1. Field of the Invention
This invention relates to methods of determining information about a subterranean formation from data obtained from a wellbore penetrating the formation. The invention more particularly relates to logging geometric information about the wellbore.
2. Description of the Related Art
When fluid produced from subterranean wellbores decreases in volume, the supporting force in the formations above the production zone also decreases. If the formation is relatively soft, formation compaction may occur. In its worst form, compaction can completely destroy the productivity of a well. In lesser forms, compaction damages various components of the well. That is, when the formation compacts, the wellbore can undergo several deformation types. In cased holes, if the casing is sufficiently strong to prevent total collapse, compaction can cause the casing within the wellbore to either expand radially, or become elliptical, or can cause large radius bends to form in the wellbore. In some cases, the largest deformation of the casing seems to occur near the casing collars. In open holes, compaction can cause fracturing, chipping and total collapse.
Early detection of small deformations which indicate a tendency toward compaction, in the life cycle of a well allows time to change reservoir management procedures so as to avoid compaction later causing major deformations and damage to the well. Casing measurements during the life of the well enable such early detection of potential compaction problems. Geometric information about the borehole is one kind of information about the wellbore casing that can be used to accomplish such measurements.
Geometric information about an uncased wellbore is also useful in detecting stress within a formation typically caused by slow plastic flow of the formation and in indicating hardness changes between adjacent beds within a formation.
While it has long been known that two way travel time of an acoustic signal through a wellbore contains geometric information about the wellbore, methods of obtaining that information have not been reported. A need exists for ways to obtain geometric information about a wellbore.